I know not every nixos-defined systemd service can run in a non-nixos system (that has systemd), But what about user services that should just work?
I’m using systemd user services provided by home-manager on Ubuntu. It “just works”.
On older systems, I had issues. Centos 7 for example, has systemd, but didn’t provide user services, so they wouldn’t work there.
I don’t think it will “just work” for nixos systemd services, but yes home-manager ones work.